When a hydrocarbon, e.g. methane, natural gas, petroleum gas, naphtha, heavy oil or crude oil, is reacted with a reforming agent, e.g. steam, air or carbon dioxide, at a high temperature in presence of a catalyst, the hydrocarbon is reformed to a synthetic gas that is a mixed gas containing carbon monoxide and hydrogen. The synthetic gas is useful as a raw material of methanol, liquid fuel, etc. Research and development have been also carried out in order to separate hydrogen from the synthetic gas in response to advancement of fuel cells in these days. Nickel/alumina and nickel/magnesia/alumina have been used so far as hydrocarbon-reforming catalysts for production of such a synthetic gas.
In a hydrocarbon/steam reacting system, a reaction by-product, i.e. carbonaceous matters, is likely to deposit on a surface of a catalyst. Once the deposition of carbonaceous matters is formed over catalytic-activity sites of the catalyst, catalytic activities are significantly reduced. Heavy deposition of the carbonaceous matters produces unfavorable results. For instance, clogging or damage of a catalyst bed, deviation of gases flowing through a reaction zone, decrease of a ratio of the effective catalyst for reforming reactions and so on. Deposition of the carbonaceous matters on the surface of the catalyst is avoided by introducing an excess volume of steam, but introduction of excess steam unavoidably increases an energy cost and needs a large-scaled plant.
A reforming catalyst, which distributes catalytic-activity sites on a surface of a carrier with high dispersion, is proposed by JP2002-126528A, in order to inhibit deposition of carbonaceous matters without introduction of excess steam. The proposed catalyst is manufactured by preparing an aqueous solution, which contains a catalytic-activity constituent, e.g. Co or Ni, together with Mg and Ca at a specified ratio, adding such a co-precipitating agent as potassium carbonate to the aqueous solution so as to precipitate hydroxides and carbonates, drying and calcining the precipitates in an oxidizing atmosphere so as to form complex oxide granules, compressing the granules to a predetermined shape, and then calcining the green compact.
The reforming catalyst proposed by JP2002-126528A is prevented from unfavorable deposition of carbonaceous matters due to distribution of catalytic-activity sites with high dispersion, but a catalytic-activity constituent is also diffused to an inner part of the carrier body. The catalytic-activity constituent in the inner part neither comes in contact with hydrocarbon or a reforming agent nor contributes to reforming reactions. In this sense, an expensive catalytic-activity constituent is wastefully consumed. Moreover, the manufacturing process comprises a lot of steps, i.e. preparation of a Mg, Ca-containing solution, co-precipitation, aging of precipitates, washing, calcining, granulation, compression and calcining. Consequently, the reforming catalyst is very expensive due to wasteful consumption of the catalytic-activity constituent and the complicated manufacturing process.
On the other hand, an impregnating and calcining process enables manufacturing a reforming catalyst at a relatively low cost, while suppressing rises of materialistic and manufacturing costs. According to this process, an oxide of a catalytic-activity constituent is supported on a carrier, as follows: A carrier is formed to a predetermined shape and soaked in an aqueous solution containing a catalytic-activity constituent. The carrier impregnated with the catalytic-activity constituent is then dried and calcined, as disclosed in JP7-206726A.
The catalytic-activity constituent, which is infiltrated into a carrier according to a conventional impregnating and calcining process, is likely to gather and scatter as island aggregates on a surface of the carrier. The island aggregation of the catalytic-activity constituent remains as such after the impregnated carrier is calcined, or rather promotes growth of the catalytic-activity constituent up to big particles due to thermal diffusion during calcining. Consequently, the catalytic-activity sites are unevenly distributed, and numerous deposition of carbonaceous matters can not be avoided.